There are many instances where it would be useful to provide the ability to easily, yet covertly, summon aid to a person in distress. A particularly poignant example is in military operations, where pilots can be shot down, foot soldiers may be cut off from their units, and similar situations.
In civilian situations, GPS-enabled cell phones can fill this need, allowing for a 911 call to both place the distress person in contact with emergency personnel as well as transmitting location information via the enhanced 911 system capabilities. Unfortunately, cell phone coverage is not always available, and GPS operation is inhibited in some situations. For example, GPS can operate poorly underground, when shielded by dense foliage, or in highly built up urban areas. Cell phones are also somewhat bulky and cumbersome to operate. Moreover, cellular coverage is not uniformly available, and can fail in disaster situations. In a hostile environment, both GPS and cellular operation can also be jammed, making them unavailable.
Further difficulties can also be presented in hostile situations. A cell phone transmission or simple distress beacon transmission can be used by both friends and foes alike to locate the person in distress, and can result in the leading of hostile forces directly to the person in distress. Equipment can fall into hostile hands, and be used for nefarious purposes (e.g., transmission of false distress signals to lure rescuers into an ambush, interception of legitimate distress signals, etc.).
Depending on the scenario, rescuers may be nearby and quick to provide aid (e.g., the separated foot soldier scenario) or rescuers may be a long distance away and unable to render assistance for some time (e.g., the downed pilot scenario). As distress signaling equipment is likely to be battery powered, low power consumption may therefore be desirable to allow for extended operation time.
Accordingly, there remains a need for emergency signaling techniques that are suitable for use in hostile environments.